It is known that catalysts of palladium on zeolite are active in hydrocracking, the acid function being obtained by the acid sites of the zeolite, the hydrogenating function by the metal. This catalyst has a very good initial activity with charges not overly contaminated by nitrogen or sulfur molecules, at temperatures as low as about 320.degree. C. However, rapid deactivation requires increasing the temperature, particularly if the pressure is lower and the charge more contaminated with sulfur and nitrogen. From thermal levels about 380.degree.-400.degree. C., the palladium catalyst deactivates very quickly and its property of good hydrogenating function disappear. This quick deactivation is partially due to the sintering of the metal phase. As a matter of fact, palladium crystallites, at high temperature (above 380.degree.-400.degree. C.) and in the presence of sulfur- and nitrogen-containing organic compounds, tend easily to couple with one another and form large particles; the metal active surface then becomes very small and the catalyst can no longer assume its action of promoting bifunctional reactions.
The other main cause of deactivation is the coking of the catalyst. Coking is a phenomenon according to which a catalyst is progressively covered with polycondensed molecules which block the access to the active sites.